The present invention is directed to coating technology and, more particularly, to pulsed detonation coating.
Several techniques have been used to implement thermal spray coating. One approach has been High Velocity Oxygen/Fuel System (HVOF), in which solid particles are injected in high velocity gas produced by reaction of oxidizer and a fuel at high pressure. Such systems typically are used for deposition at atmospheric pressure and primarily are used for coating metal alloys and WC/Co powders with particle sizes larger than about 10xcexc. Other thermal spray coating techniques include plasma spray, in which particles are heated and accelerated by high temperature plasma produced by an electric discharge in an inert gas atmosphere. Plasma spray systems have been used for both atmospheric- and low-pressure coatings.
Thermal spray coating also has been implemented by intermittent detonations, e.g., by the use of a detonation gun (D-Gun). D-guns can be used for coating a wide variety of materials, such as metals, cermets, and ceramics. D-guns typically have a relatively long (often about 1 m), fluid-cooled barrel having an inner diameter of about one inch. Typically, a mixture of reactive gases, such as oxidizer and acetylene, is fed into the gun along with a comminuted coating material in two phases. The reactive gas mixture is ignited to produce a detonation wave, which travels down the barrel of the gun. The detonation wave heats and accelerates the coating material particles, which are propelled out of the gun onto a substrate to be coated.
The detonation wave typically propagates with a speed of about 2.5 km/sec in the tube and can accelerate the particle-laden detonation products to a velocity of about 2 km/sec. However, coating particles never reach the velocity of detonation products due to inertia. In practice, particle velocities usually are lower than about 900 m/sec. The temperature of the detonation products often reaches about 4000 K. After the coating material exits the barrel of the D-gun, a pulse of nitrogen typically is used to purge the barrel. Newer designs of the D-guns allow operation frequencies of up to about 100 Hz. See, e.g., I. Fagoaga et al., xe2x80x9cHigh Frequency Pulsed Detonation (HFPD): Processing Parametersxe2x80x9d (1997).
One example of a gas detonation coating apparatus is illustrated in U.S. Pat. No. 4,669,658 to Nevgod et al. A barrel enclosed in a casing has annular grooves made on an inner surface of an initial portion thereof. A main pipe housing a spark plug and having annular grooves on its inner surface is inserted into the initial portion of the barrel. In operation, a gas supply means is turned on. The apparatus works in cycles, each cycle accompanied by gas flowing into the barrel and the main pipe through tubes, gas conduits, and additional pipes. After the gases fill the barrel, the gas mixture is ignited in each cycle with the aid of the spark plug. The detonation products are said to quickly heat up the walls of the barrel and the annular grooves.
According to Nevgod, the gases flowing into the barrel are heated up in two stages. During the first stage the gases are warmed up in the additional pipes heated up in cycles by the detonation products. The heat insulation tubes are said to prevent the pipes from cooling down. During the second stage, the gases are heated up in the barrel and partially in the main pipe. The annular grooves on the inner cylindrical surface of the initial portion of the barrel, the inner surface of the main pipe and on the inner surface of the cover on the end of the barrel, are said to enhance the efficiency of heat exchange with the gases due to an increase in the heat exchange area and due to gas turbulization. The gases are heated to a temperature approximating that of self-ignition. A plurality of ignition sites is provided to accelerate the burning process.
Presently available detonation coating devices suffer from several drawbacks. One major drawback is that the devices are implemented at a scale that makes coating of internal surfaces and difficult-to-reach spaces difficult, if not impossible. For example, present D-guns are unsuitable for coating the internal surface of tubes having an internal diameter smaller than about 10 cm. One HVOF system recently was modified to enable coating inside of a cylinder borehole of an engine block. This was done to permit coating of metallic materials over ceramic surfaces of the cylinder bores. However, because of the size limitations inherent to HVOF systems (e.g., the need for a cooling jacket, the relatively large nozzle diameter, the large size combustion chamber and the bulky gas feed lines), this technology is unsuitable for coating the inside surfaces of long, small-diameter tubes. In addition, large particles used in HVOF systems need substantial distances to achieve the velocities and temperatures needed for producing coatings having an acceptable quality.
There remains a need for technology for coating difficult-to-reach surfaces, particularly for applying coatings that provide protection against corrosion, erosion, and wear. It would be desirable to develop thermal spray coating technology that enables the use of a miniaturized coating apparatus, especially one that can be adapted for coating difficult-to-reach surfaces, especially the inside surfaces of small-diameter tubes, the inner surfaces of cylinders, the inner surfaces of converging/diverging shapes, the inner surfaces of small rectangular tubes, the inner surfaces of shapes that are partially open, and the inner surfaces of various other non-cylindrical shapes.
The present invention is directed to a method and apparatus for producing a coating on a substrate using a pulsed detonation gun. According to one embodiment, a pulsed detonation gun comprises a small-diameter detonation tube, an igniter, and an outlet for discharging detonation products. A detonable mixture containing a coating precursor is formed in the detonation tube, and the detonable mixture is ignited to produce detonation products containing the coating precursor. The coating precursor is discharged through the nozzle and is contacted with the substrate to produce a coating.
In another embodiment, a coating material is produced in situ by reaction of a coating precursor in a detonable or reactive mixture. The coating precursor is intermittently injected into the detonation tube together with other component(s) (e.g., fuel and oxidizer) to form a detonable or reactive mixture. The detonable or reactive mixture is ignited to produce high-temperature detonation or reaction products, e.g., during a detonation process or a deflagration process. The detonation or reaction products heat and accelerate the coating material through the detonation tube and toward the surface to be coated.
The pulsed detonation gun of the present invention is particularly useful for directly depositing coating material(s) over internal surfaces of tubes and other hard-to-reach surfaces of a substrate, such as the inner surfaces of cylinders, the inner surfaces of converging/diverging shapes, the inner surfaces of rectangular tubes, the inner surfaces of shapes that are partially open, and the inner surfaces of various other non-cylindrical shapes.
The material(s) are deposited by high velocity gas products produced in intermittent detonations or an intermittent injection and deflagration process. The detonation tube and its associated fuel/oxidizer supply lines can be constructed at a sufficiently small scale that allows their insertion into long, small-diameter tubes, and permits their use in coating various other difficult-to-reach surfaces. Of course, the apparatus of the present invention also is useful for coating a wide variety of large-diameter tubes, such as gun barrels, tubes used in oil industries, tubes used in food industries, etc.
The detonation products produced by the pulsed detonation gun accelerate and heat the coating precursor or coating material particles to high kinetic energies, resulting in high quality coating depositions that can provide such properties as corrosion-, erosion-, and wear resistance. Existing thermal spray coating equipment is unsuitable for applying such coatings to the inner surfaces of small-diameter tubes and many other difficult-to-reach surfaces.
The pulsed detonation coating device of the present invention avoids the need for forced water- or air-cooling because the internal volume of the device is exposed to the relatively cold injected gases between the occurrences of detonations, which are characterized by very short periods of high pressure and temperature conditions. The device can be operated with a relatively low gas line pressure and does not require a separate combustion chamber. These factors each contribute to the ability of constructing the device at scale substantially smaller than is possible with presently available technologies.
According to another embodiment of the present invention, a method for producing a coating on a substrate comprises providing a pulsed detonation gun comprising a detonation chamber, an igniter, and an outlet for discharging detonation products. A coating precursor is mixed with fuel or oxidizer, such as by forming a suspension of coating precursor particles in fuel or oxidizer, prior to injection into the detonation chamber. The detonable mixture is ignited to produce detonation products containing the coating precursor. The coating precursor is contacted with the substrate to produce a coating on the substrate.